Homeostasis of inner ear fluids is crucial for sensory transduction and consists of the regulation of ion and water transport across barriers formed by epithelial and endothelial cells. Pathology of these homeostatic processes are thought to lead to endolymphatic hydrops and edema in stria vascularis. Epithelial cells in both the cochlea and the vestibular labyrinth of the inner ear are responsible for the secretion of potassium (K+), the primary cationic constituent of endolymph. It is expected that water would follow this secretion by osmotic forces and that it is therefore necessary for K+ to be reabsorbed from endolymph at exactly the same average rate in order to avoid catastrophic changes in the volume of endolymph. We propose to test the hypotheses a) that the epithelial tissues responsible for K+ secretion,can also reabsorb K+ when necessary to augment the K efflux through the sensory hair cells, thus maintaining endolymph volume; b) that the K+-secretory process is regulated by extracellular influences such as osmolarity and K+ concentration and that these act via cytosolic regulatory pathways; c) that the flux of water accompanying K+ transport is limited by the water permeability of the apical membrane and that this water permeability is under cytosolic regulation; d) that K+ secretion in the cochlea is controlled the cytosolic pH, as in the vestibular labyrinth, - and that a variety of mechanisms is involved in pH regulation; e) that vestibular transitional cells and spiral prominence cells provide another homeostatic route for the reabsorption of endolymphatic K+ and/or Na+; f) that capillary blood vessels in the cochlear lateral wall (spiral ligament and stria vascularis) and in the ampullae of the semicircular canals are part of the endothelial barrier analogous to the blood-brain barrier and this barrier is under the control of hormones and local mediators. Specific parameters to be measured include microfluorometry of pH- and calcium-sensitive dyes, cell volume and capillary permeability/diameter with digital imaging techniques, transepithelial voltage and resistance with the micro- Ussing chamber, transepithelial fluxes of K+ and Na+ with the ion- selective vibrating probe and electrical properties of cell membranes with several configurations of the patch clamp technique. These methods are well-established in this laboratory and represent powerful techniques for addressing the proposed hypotheses.